FIG. 12(A) is a block diagram illustrating the configuration of a conventional electromagnetic flowmeter. The electromagnetic flowmeter is configured from a detector 1 and a converter 2. The detector 1 is configured from an excitation coil 10 that generates a magnetic field, and a measurement tube 11 that is arranged in the magnetic field generated from the excitation coil 10, and that detects an electromotive force generated by a measurement-target fluid flowing through this magnetic field, and outputs a flow-rate signal that is proportional to the flow velocity thereof. The converter 2 supplies an excitation current such as that illustrated in FIG. 12(B) to the excitation coil 10 of the detector 1, and converts a flow rate signal such as that of FIG. 12(C) input from the detector 1 into an analog signal or a digital signal that indicates the flow rate or flow velocity of the fluid.
The flow-rate signal that is input from the detector 1 to the converter 2 is a very small μV-order signal, and therefore measurement precision is liable to deteriorate due to changes that occur with the aging of the electrical components used in the converter 2. Thus, a standard signal generator (hereinafter, a calibrator) is used at the site where the electromagnetic flowmeter is installed to periodically perform calibration as described below (see Patent Document 1).
In the calibration work, first, instead of the detector 1, a calibrator 3 having a configuration such as that illustrated in FIG. 13(A) is connected to the converter 2. The calibrator 3 is configured from: an input circuit 30 that receives an excitation current such as that of FIG. 13(B) input from the converter 2; a central processing unit (CPU) 31 that generates a reference flow-rate signal; an output circuit 32 that outputs the reference flow-rate signal generated by the CPU 31; a setting/display device 33 for setting the calibrator 3 and displaying information to a calibration worker; a power source circuit 34; and a battery 35. The calibration worker uses the setting/display device 33 to set, in the calibrator 3, information regarding the model of the converter 2 and a flow velocity value at a calibration point.
The CPU 31 of the calibrator 3 outputs a reference flow-rate signal corresponding to the set flow velocity value in synchronization with an excitation current that is input from an XY terminal of the converter 2 by way of the input circuit 30. This reference flow-rate signal is input to the converter 2 as a signal such as that illustrated in FIG. 13(C) by way of the output circuit 32. The calibration worker confirms data that is output from the converter 2 in accordance with the reference flow-rate signal, and confirms whether or not the measurement precision of the converter 2 is within a permitted range. If necessary, the converter 2 is readjusted in accordance with this confirmation result.
It is necessary for one calibrator 3 to correspond to a plurality of models of converters 2; however, there are a variety of excitation currents according to the models. In a standard type of four-wire electromagnetic flowmeter, the excitation current is of the order of ±100 to 200 mA; however, in a fluid-noise countermeasure type of electromagnetic flowmeter that is used for fluids in which solid bodies such as paper pulp are mixed, an excitation current of ±300 mA or greater is passed to improve the S/N ratio. In contrast, in a two-wire electromagnetic flowmeter in which there is a limit to the current that can be used, the excitation current is of the order of ±3.5 to 12 mA (see Patent Document 2). If the excitation current differs, the flow-rate signal level also differs even if the fluid flow velocity is the same, and therefore it is necessary for the calibrator 3 to output a flow-rate signal that corresponds to the model of the converter 2 and a set flow velocity value.
Furthermore, there is a decrease in the average current consumption value in two-wire electromagnetic flowmeters and battery-type electromagnetic flowmeters, and therefore there are also types in which an excitation current pause period (=0 mA) is provided (see Patent Document 3). It is necessary for the flow-rate signal output by the calibrator 3 to also be set to zero during this pause period.
It is necessary for the calibrator 3 to be small and lightweight such that work can be easily carried out at the installation site, and therefore it is not possible for large components such as the excitation coil of the detector 1 to be housed therein. Thus, the input circuit 30 of the calibrator 3 is an extremely simple circuit such as that illustrated in FIG. 14. To be specific, diodes D100 and D101 that are connected in parallel in opposing directions and a parallel resistor R100 are components that are provided instead of the excitation coil of the detector 1, and an excitation current Iex flows through these components. A voltage VAD that is unipolarized with an offset voltage being added to the voltage Vxy across both ends of these components (XY interterminal voltage) due to resistors R101 and R102 and a capacitor C100 is input to an A/D converter housed inside the CPU 31, and polarity changes and pause periods of the excitation current Iex are detected.
FIGS. 15(A) to 15(C) illustrate examples of operation waveforms when the calibrator 3 is connected to the converter 2 (excitation current Iex is ±150 mA) of a four-wire standard type of electromagnetic flowmeter. FIG. 15(A) illustrates the excitation current Iex, FIG. 15(B) illustrates the XY interterminal voltage Vxy of the converter 2, and FIG. 15(C) illustrates the output voltage VAD of the input circuit 30.
Since the input circuit 30 has the configuration illustrated in FIG. 14, even when a large excitation current Iex flows to the input circuit 30 as in the case where the converter 2 of a four-wire standard type of electromagnetic flowmeter or the converter 2 of a fluid-noise countermeasure type of electromagnetic flowmeter is connected to the calibrator 3, the XY interterminal voltage Vxy is suppressed to less than ±1 V due to the IF-VF (forward current-forward voltage) characteristics of the diodes D100 and D101, and therefore the diodes D100 and D101 and other internal components do not generate heat.
Furthermore, in the case where the excitation current Iex is small as in when the converter 2 of a two-wire electromagnetic flowmeter is connected to the calibrator 3, the diodes D100 and D101 enter a near-high impedance state, and therefore the output voltage VAD of the input circuit 30 exhibits near-linear characteristics and it becomes possible for polarity changes and pause periods to be detected. FIG. 16(A) and FIG. 16(B) illustrate the characteristics of the XY interterminal voltage Vxy and the output voltage VAD when the excitation current Iex is made to change from −300 mA to +300 mA as indicated by the horizontal axis.
In calibration work in which the calibrator 3 such as that described above is used, there are cases where a commercial power source is not able to be obtained at the installation site of the electromagnetic flowmeter, and in order for calibration work to be possible even at such sites, the battery 35 is used as a power source for the calibrator 3, and the voltages required for each section of the calibrator 3 are generated by the power source circuit 34.